Week 3

Last week you learned about callbacks - functions that run later. You saw how nesting callbacks gets messy ("callback hell"). This week, you'll learn Promises and async/await - cleaner ways to handle asynchronous operations.

This week you'll work with a real API: the Tea Shop backend. You'll fetch data, handle errors, and chain operations - just like you would when building real backend services.

Contents

• Preparation

• Slides

• Session Plan (for mentors)

• Session Materials

• Assignment

Learning Goals

By the end of this session, you will be able to:

• Understand why Promises exist (solving callback hell)

• Consume Promises with .then() and .catch()

• Chain multiple .then() calls

• Create your own Promises with new Promise()

• Use async/await syntax for cleaner code

• Handle errors with try/catch in async functions

• Run multiple Promises in parallel with Promise.all()

• Fetch data from APIs using fetch()

After this session you will understand code like this

// From callback hell...
validateOrder(order, (err, valid) => {
  if (err) return handleError(err);
  calculateTotal(order, (err, total) => {
    if (err) return handleError(err);
    checkStock(order, (err, inStock) => {
      if (err) return handleError(err);
      // deeply nested...
    });
  });
});

// ...to clean async/await
async function processOrder(order) {
  const valid = await validateOrder(order);
  const total = await calculateTotal(order);
  const inStock = await checkStock(order);
  return { valid, total, inStock };
}